The present invention relates to a method of adjusting power for a wavelength-division multiplexed (WDM) optical transmission system.
Digital optical channel transmission, in particular over an amplified link, is limited firstly at low powers by noise and secondly at high powers by non-linear effects.
In any amplified link, there exists an optimum outlet power level from each of the amplifiers which ensures the lowest possible error rate.
For transmission without wavelength-division multiplexing, the optimum power can be adjusted by the power of the pump in each amplifier.
However, when the transmission uses wavelength-division multiplexing, it is no longer possible to adjust the gain of each of the signals individually, since the response of the amplifiers is not flat with varying wavelength.
Three main techniques have been described for resolving this problem.
One of those techniques consists in optimizing the dopants of the amplifying fiber and in optimizing mean population inversion: flatness of about 1.5 decibels (dB) over 30 nanometers (nm) can thus be obtained for gain of 25 dB.
In this respect, reference can be made to the following publication:                K. Inoue, T. Kominato, H. Toba, IEEE Photon. Techn. Letters 3, 718 (1991);        
Sulhoff, Smart, Zyskind, Nagel, DiGiovanni, “Gain peaking in concatenated 980-nm-pumped EDFAs”, OFC'94, p. 40 (1994).
Another technique consists in using optical filters.
In general, those first two techniques are used for the purpose of limiting variations in gain to a few tenths of a decibel.
A third technique consists in pre-emphasizing the signals, i.e. in reducing inlet power at those wavelengths that have higher gain and in increasing inlet power of signals for which gain is lower. Pre-emphasis is considered to be optimized when the signal-to-noise ratios in the various channels are equal.
A technique of that type is described in particular in:                Chaplyvy, Nagel, Tkach, “Equalization in amplified WDM lightwave transmission systems”, IEEE Photon. Techn. Lett. 4, 8 (1992);        Chraplyvy, Tkach, Reichmann, Magill, Nagel, “End-to-end equalization experiments in amplified WDM lightwave systems”, IEEE Photon. Techn. Lett. 4, 428 (1993).        
Nevertheless, that technique cannot correct cumulative gain differences of more than 25 dB. In addition, it increases the non-linear penalty for given signal-to-noise ratio.
Furthermore, one of the main sources of degradation in a WDM terrestrial transmission system having amplifiers with equalized gain is the way the gain characteristic curve varies as a function of temperature. This variation is about 1 dB per 25 dB of gain over a temperature range of 50° C. Variation in line losses is another factor that degrades the flatness of amplifier gain.
To resolve that problem, proposals have been made, in particular in European patent application No. 0 580 497, for a device which adjusts the source parameters as a function of the error rate on reception, as estimated or as measured.
Nevertheless, information concerning error rate is not always available on reception.
Proposals have already been made, in particular in Japanese patent abstract No. JP 08 223136 for techniques which consist in equalizing gains at each of the amplifiers in a transmission line.